I've had this receiver for a number of years and am quite happy with it overall.
A bit of usage history.
I use my receiver for AM/FM listening, CD's as well as movies and TV. I run everything through it.
A few months ago while watching TV and Movies (DVD,VHS) the sound started switching on and off randomly, however the music side is still working properly. you could actually hear the part inside switch on and off.......If I knew what part it is I could replace it.....
What you are probably hearing switching on and off, is the speaker protection relay, but it is unlikely to be because the relay itself is faulty. Although in saying that, I did have one a few months back where a very similar problem was being caused by the main power transformer control relay having burnt contacts. It would randomly click, and drop back to standby, and then go through the power up sequence again, sometimes successfully, sometimes not. I have also had bad joints on relay coil connections for both standby and output protection relays.
I don't really understand what you mean by " the music is still working properly ". If you mean that any source going through the amp carries on uninterupted, then it will, no matter what the amp is up to. Assuming that the amp does not appear to be actually switching off, then I think what you are hearing is probably the output protect relay doing its job. This could be for one of two basic reasons. Either the amp is faulty, and some level of DC is appearing on one of the output channels or, one of the speakers is faulty, and on heavy bass, is imposing an abnormal load on the output, which the protection circuitry will also sense. I have had this on many occasions with many makes and models. You can try disconnecting the speakers one at a time, and if this seems to effect a cure, go on to swap them. If it is not the speakers, then without in-depth experience of this type of equipment, you will struggle to fix it and will have to take it to a repair shop.
I don't think I was overly thorough in explaining what it does... The whole unit doesn't power off, the sound just clicks on and off but only when the signal is pumped out throught the Monitor setting. The system works fine throught the tuner side...as well as through the CD player is fine.
I've never been a great lover of TV sets connected to audio gear. Far to many iffy grounds at odd potentials. As the system apparently works ok from the CD, you could try disconnecting the TV set from the input sockets you are using, and connecting the CD player instead. Go back into monitor mode, and see if the system still cuts in and out with a CD playing. That will at least tell you if it is a definite amp issue, or something external.
To go back to the start. The click you are hearing is probably the output protect relay, which is not controlled as a result of operating any front panel switches, per se. As has been suggested, a relay in this position might drop in and out as a result of bad switch contacts, if it is a mechanical switch. Such bad contacts can cause a small DC shift in the early stages of the amp, which becomes a big shift at the output, and is hence detected by the circuit which drives this relay, as a fault. However, with your selector switch being a tact type, this is an unlikely scenario for this case.
If you normally get a quiet click from inside, when you operate this switch, then there may be a small signal relay in the path, and this may be dropping in and out for whatever reason, but if you hear a loud mechanical click from inside the unit when the problem occurs, then it is a power relay that is opening and closing, not a small signal one. Either way, from the replies that you have made to the various suggestions, I would guess that this problem is going to turn out to be beyond your level of expertise, if it is indeed internal to the amp, so your best path would be to take it inro a reputable repair shop.